Shear stress is defined as the component of stress that is coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section and is defined as force applied divided by the cross sectional area of the deformed material with area parallel to the applied force. The applied force is often referred to as the shear, or lateral force.
There is a large class of sensing applications that require the measurement of lateral displacements or forces. Ideally, these measurements should be with a small planar sensor that is robust and capable of measuring the lateral displacements or forces while simultaneously withstanding vertical loading. It is also desirable to have minimal cross coupling between the desired lateral force measurement and loading in the vertical direction. In certain applications it is also desirable to simultaneously measure the lateral and vertical forces.
A particularly interesting practical application for a planar lateral load sensor is in wearable motion capture. Human movement is the result of complex inter-dependent components that include joints, skeletal, neuro and muscular components. The study of human movement kinematics is usually undertaken by motion capture (mocap) sensor systems. Motion tracking technology is routinely used in biomechanical research, sports training, medical assessment, digital animation, virtual reality and computer/gaming interfaces to capture user biomechanical movements. Mocap systems are usually based on optical tracking, magnetic tracking, inertial sensors, ground-reaction force measurement, or combinations of these sensors (W Tao, T Liu, R Zheng and H Feng, Review: Gait Analysis Using Wearable Sensors, Sensors 2012, 12, 2255-2283).
Ground reaction force (GRF) is a vector that is the result of contact and can be measured using instrumented forceplates. The center of pressure (COP) is a related measure corresponding to the point of application of the ground reaction force vector. Analysis of COP is common in studies on human postural control and gait. It is thought that changes in motor control may be reflected in changes in the COP.
COP can be measured using instrumented mats (for example, Gait Mat II) and in instrumented shoes (for example, M3D, Xsens (ForceShoe), Pressure Profile (3D TruCapture) and TekScan (F-Scan VersaTek Wireless)). COP measured within an instrumented shoe will be specific to the associated limb; usually the COP for the left and right foot will be separately measured (together with the weight or load for each foot). If there is simultaneous weight on both shoes (i.e. the user has both feet in contact with the ground for example when standing or during aspects of gait), the COP of the user can then only be estimated by using the COP measured for each shoe and also an estimate of the location of the shoes.
Ground reaction force can be approximated by pressure or determined from the combination of pressure and shear using standard biomechanical models (D. Lafond, M. Duarte, F. Prince, Comparison of three methods to estimate the center of mass during balance assessment, Journal of Biomechanics 37 (2004) 1421-1426). Combination sensor systems have been proposed (for example by Xsens and ShapeWrap 3), which include six degrees of freedom force and moment sensors and miniature inertial sensors to estimate joint moments and powers of the ankle (Martin Schepers, Ambulatory Assessment of Human Body Kinematics and Kinetics, Thesis University of Twente, 2009). The combination of 3D motion sensors and GRF sensors in shoes has also been proposed as a clinic gait analysis human assistance tool.
Foot pressure (load) sensor designs based on capacitive (M Cheng et al. A Polymer-Based Capacitive Sensing Array for Normal and Shear Force Measurement, Sensors 2010 (10) 10211-10225), optical and piezoelectric sensors have been proposed but do not usually provide measurements of shear. Studies have shown (Kuo, A D. (1998)) a least squares estimation approach to improving the precision of inverse dynamics computations (J Biomech Eng 120:148-159). COP information may be sufficient to estimate shear forces using a least squares inverse dynamics (LSID) approach. However, this is computationally intensive (i.e. prone to delay) as the complete model must be known. Innovative instrumented shoes measure the shear directly using multi-axis load cells under the shoe (H. M. Schepers, et al. ‘The Forceshoe’: What Has Been Achieved?—Ambulatory Estimation of Ankle and Foot Dynamics and Center of Mass Movement. The 10th International Symposium on 3D Analysis of Human Movement, Oct. 28-31, 2008). However, this design is bulky, adds significant height to the shoe platform, and restricts the user to wearing specially designed and instrumented shoe.
There is an unmet need for an instrumented shoe that fits an insole form factor i.e. flexible and less than 5 mm thickness. This invention describes a sensor and approach for measuring lateral (shear) and vertical loads in a planar form factor that can fit within an instrumented insole in a shoe or in other locations, such as between layers of protective clothing, between clothing and the skin, between a held object and the hand, and others.
There are significant advantages to measuring in-shoe COP and loading (weight); the state classifier and process for discriminating between sitting, standing and walking activities is relatively simple, and the instrumented insole sensor can be potentially used to determine the ground reaction force and together with other segment data, the stability margin. Further, shear measurement (together with COP) is also a natural and rapid indicator of the various gait cycles including initiation and termination of gait. Shear is also a potentially sensitive measure of the onset of slip (and potential balance instability).
The foregoing information reflects the current state of the art of which the present inventor is aware. Reference to, and discussion of, this information is intended to aid in discharging Applicant's acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above information discloses, teaches, suggests, shows, or otherwise renders obvious, either singly or when considered in combination, the invention described and claimed herein.